The subject of the present invention is a unidirectional tape of carbon fibers.
It is common practice in the composites industry to use carbon fibers because of their reinforcing capability and their lightness. Carbon fibers able to be used fall within various grades that may be defined as follows, depending on their Young""s modulus:
These fibers may be obtained from precursors, such as polyacrylonitrile (PAN), pitch or rayon.
A first solution consists in using continuous carbon filaments having the same orientation, which are impregnated with thermosetting resins. These continuous filaments form ribbons or tapes, oriented in a particular way depending on the stresses to which the final component will be subjected. These tapes may be used singly or stacked one on top of another after having been cut beforehand to the dimensions of the component to be produced. The relative orientations of the filaments in the tapes may vary from xc2x10 to 45xc2x0, xc2x10 to 30xc2x0 or xc2x10 to 60xc2x0. All angular combinations are possible, both symmetric and asymmetric ones. The preforms employed are then consolidated, by cross-linking the resin by raising the temperature in an autoclave, using either closed molds or membrane devices, in which the preforms are under vacuum.
Difficulties are encountered in these various systems when the components include substantial projections or indentations. This is because the continuous carbon filaments cannot slide relative to one another. Consequently, some of the filaments break or folds are formed.
The same difficulties are encountered in the technology of producing carbon-carbon components obtained using either a wet route or a gas route.
In carbon-carbon materials, the reinforcing fibers are made of carbon and the matrix is a carbon substrate deposited between the fibers and bonded to the latter. The carbon substrate is obtained by carbonizing phenolic resins in an inert atmosphere or under vacuum, these being deposited beforehand during the initial impregnation of the unidirectional carbon tapes. The carbonization comprises a certain number of steps in order to end up with the correct densification (decrusting, reimpregnation, etc.). This constitutes the wet technique.
The gas technique consists in starting with unimpregnated unidirectional tapes, draped beforehand in a mold porous to gases. A gas stream, consisting of a saturated hydrocarbon, for example methane CH4, and which has undergone, before flowing through the preform, a cracking operation which allows the carbon atoms to be separated from the hydrogen molecules: CH4xe2x86x92C+2H2, flows through the oven. The carbon atoms are trapped in the fibrous preform and constitute, after a relatively lengthy time, interspersed by decrusting operations, the solid matrix which bonds the fibers of the preform together.
It is also known, from European Patent EP-A-274,970, to convert continuous carbon filaments into discontinuous fibers, by slow controlled drawing, using a cracking technique. The fibers obtained are of variable, but controlled, length. After obtaining parallel fibers, these fibers are subjected to a conventional spinning operation. However, in order to carry out a spinning operation, it is necessary to have fibers whose mean length is between 90 and 120 mm approximately. Under these conditions, it is not possible to produce components of complex shape and having very deeply indented or projecting features since if the fibers move significantly with respect to one another a dislocation of the tape is produced. Conversely, if a component having numerous but small deformations has to be produced, the fibers are too long to allow the tape to follow such deformations.
The object of the invention is to provide a unidirectional tape of carbon fibers, which can be deformed in order to produce deeply indented shapes or to produce a plurality of small raised features. For this purpose, in the tape to which the invention relates, the fibers are discontinuous, have a length distribution such that the mean fiber length, that is to say the mean length of 50% of the fibers in a specimen, is between approximately 40 and 70% of the length of the longest fiber in the specimen, and the fibers are combined by means of a resin or of an adhesive having a fugitive nature.
Since the fibers are discontinuous, they can slide relative to one another during the forming step for producing a component, after the viscosity of the resin has been lowered or the adhesive having a fugitive nature has allowed the fibers to be released. Advantageously, this tape has an extendibility corresponding to a relative displacement of the carbon fibers within a range of between 0 and 45% of the longest fiber.
According to one characteristic, the mean fiber length is greater than 30 mm.
According to one option, the mean fiber length is greater than 120 mm.
According to another option, the mean fiber length is between 30 and 90 mm. It is apparent from the foregoing that the mean fiber length may vary over a very wide range, making it possible, according to applications, to have fibers whose mean length is large or, on the contrary, fibers whose mean length is small.
The tape according to the invention may be produced with fibers obtained from carbon multi-filaments chosen from among all the grades defined above. The tape may have a width of, for example, between 10 and 200 mm, depending on the choice of linear density of the multifilaments as well as on the number of multifilaments which are juxtaposed in order to obtain the desired width.
The linear densities of the starting multifilaments may be chosen from the following values, where K=103: 400 K, 320 K, 160 K, 80 K, 70 K, 60 K, 50 K, 40 K, 24 K, 12 K, 6 K, 3 K.
According to one embodiment of the tape according to the invention, the fibers are combined by impregnating them with a thermoset resin chosen from epoxy, phenolic, bismaleimide and furan resins.
According to another embodiment of this tape, the fibers are combined by spraying a cellulose-based spray.
Next, the carbon-fiber assembly is coated, on both its sides, with a protective film, for example a polyethylene film. This film protects the assembly and makes it easier to handle, especially allowing it to be stored in a position wound on a mandrel.
A process for producing this tape consists, starting from a series of continuous and parallel carbon filaments, in cracking the filaments by slow and controlled drawing, inside a cracker, and then in combining the discontinuous fibers in line, without deforming the tape and without exerting tension on the latter, by means of a resin or of an adhesive having a fugitive nature.
Depending on the characteristic of the multifilaments taken into the cracking machine and depending on the desired mean length for the fibers of a specimen, it is possible to adapt the characteristics of the machine. Thus, in order to obtain long fibers, it is necessary to move the series of tensioning rollers further apart and to have a relatively small speed difference from one series of rolls to the next whereas, in order to obtain shorter fibers, the distance between the various series of tensioning rolls must be reduced and the speed difference from one roll to another must be increased. These adjustments must also take into account the linear density and the grade of the carbon filaments.
According to a first method of implementation, this process consists in combining the discontinuous carbon fibers by resin impregnation, by passing them through a bath.
According to another method of implementation, this process consists in combining the discontinuous carbon fibers by spraying a cellulose-based spray onto both sides of the tape.
In either case, this process consists, after the discontinuous carbon fibers have been combined, in evaporating the solvents.
Finally, this process consists, after the discontinuous carbon fibers have been combined, in depositing two protective strips, for example made of polyethylene, on both sides of the tape, before it is wound on a mandrel.